It is currently a common practice to embed switches and sensors in upholstered seats, such as automobile seats to perform various control functions, such as to detect the presence of a driver or passenger in the seat, to sense the size and/or weight of a person in the seat, etc. Because of the flexible nature of the mediums involves, it is convenient to employ flexible membrane circuit elements, which can accommodate the flexing and distortion of the upholstery fabric and padding when a person occupies a seat.
Conventional membrane circuits are commonly constructed by printing a conductive ink onto a surface of a plastic film in the form of circuit elements. However, conventional membrane circuits are not a very suitable medium for use in connection with fabric, vinyl, or leather upholstery because they have the disadvantage of being able to bend in only two dimensions, while fabric, vinyl or leather upholstery will bend in three dimensions. Some attempts have been made to avoid the described disadvantage of plastic film membranes by coating a conductive matrix directly onto the surface of a suitable fabric in wide web roll form and later die cutting the fabric into strip sections which can be joined in a desired circuit pattern and bonded to the interior of an upholstery fabric. While such arrangements are more suitable from an aesthetic standpoint than the plastic film based membrane circuits, they are rather labor intensive and very costly, because of the die cutting waste generated as a by product of the die cutting process. Conductive fabric waste will be 50% or more, and intensive labor is required to attach the die cut strips to the inside of the upholstery. Particularly for automobile usage, for example, where several sensors may be employed, the high cost of the known fabric-based circuits is a serious disadvantage. If the conductive fabric is coated on its opposite side with a pressure sensitive (PSA) , the installation still generates a 50% plus waste of the conductive fabric, although the (PSA) will reduce the labor required to attach the die cut sections to the upholstery material.
In accordance with the present invention, a novel and improved fabric-based flexible circuit is provided, in which the circuit substrate is a tightly woven polyester or nylon taffeta fabric. A conductive ink formulation is applied to one surface of the fabric in a predetermined circuit pattern, using screen printing procedures. The opposite surface of the fabric is coated with a pressure sensitive adhesive (PSA) or a heat activated adhesive for easy application to upholstery. Standard well known screen printing methods are employed to apply the conductive ink only in the graphic form that makes up the conductive circuit. The printed circuit pattern can be applied in multiple repeats on a fabric section, and the fabric section later die cut into its individual component areas. In this way, the expensive conductive ink formulation is not wasted when the circuit is die cut. The waste generated by the die cutting process will only contain the base fabric, leaving the conductive circuit portions intact on their carrier fabric sections for later application to the upholstery. The circuit elements can then easily be installed in an upholstered seat, for example, by adhesive bonding to the underside of the upholstery structure, whether the inner surface of the outer upholstery fabric or a liner of foam or fabric, for example. The fabric based circuit elements can flex and distort with the seat material and are substantially undetectable by the eye or by touch. The circuit elements can be easily installed by adhesive techniques, either by simple contact, with pressure sensitive adhesive, or by heat activated adhesives where heat is employed to soften the adhesive for bonding to the upholstery. Bonding is achieved when the heat activated adhesive cools and the adhesive returns to its original solid state.
Sensor circuit arrangements employing the new circuit elements of this invention, in addition to being functionally advantageous, are significantly less expensive to make and install than known types of fabric-based circuit elements.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment of the invention and to the accompanying drawing.